Answer:
During the heating of magnesium in air, it gains mass with the addition of oxygen present in air to form the oxide. However, during the heating of potassium manganate (vii), it decomposes or breaks down to simpler products and oxygen gas is given off.
Step-by-step explanation:
During the heating of magnesium in air, it gains mass with the addition of oxygen present in air to form the oxide.
The equation for the reaction is given below:
2Mg(s) + O2(g) ---> 2Mg0
The molar mass of the magnesium metal is 24.0 g while the mass of the magnesium oxide is 40.0 g. The total mass of the metal before heating is 24 × 2. + 32 = 48.0 g. The total mass after heating = 48 + 32 = 80.0 g. Therefore, there is an increase in mass.
However, during the heating of potassium manganate (vii), it decomposes or breaks down to simpler products and oxygen gas is given off. The equation for the reaction is given below;
2KMnO4(s) ----> K2MnO4(s) + MnO2(s) + O2(g)
With the evolution of oxygen gas, there's a loss in mass of potassium manganate (vii). Molar mass of KMnO4 is 158.0 g while the mass of K2MnO4 is 197.0 g and that of MnO2 is 87.0 g. The total mass on the right hand side is 2 × 158 = 316.0 g. The total mass on the left-hand side with the exception of the gas evolved is 284.0 g. Therefore, there is a loss in mass.